The invention relates to an anti-skewing device for a crane having a bridge with a set of wheels near opposite ends thereof.
Travelling bridge cranes, including gantry cranes and overhead cranes, travel along spaced-apart rails rear opposite ends of the bridge. A problem commonly arises where one end of the bridge lags behind the other as the crane travels along the rails. This condition is referred to as "skewing" and is undesirable because of the resulting stresses on the crane structure and the hinderance to the smooth travel of the crane along the tracks. This problem could eventually cause structural failure of the crane or possibly the derailment of the crane.
Because of this problem, various mechanisms have been developed to retard the leading end of the crane relative to the opposite end so that the proper condition is restored where the two ends of the crane are at the same longitudinal position with respect to the tracks. For example, U.S. Pat. No. 3,166,023 to Lynd discloses an anti-skew device for an overhead crane wherein rollers detect skewing of the crane. The rollers are connected to switches and a circuit arrangement whereby the skewing of the crane is corrected by frictional drag. In U.S. Pat. No. 1,181,324 to Lent, forked extensions of operating members detect the skewing. The operating members activate switches controlling the motors of the crane. An overhead crane with skew control is shown in U.S. Pat. No. 3,204,577 to Smith which includes a bridge skew detector carried by the bridge and a motor controller sensitive to the skew detector for adjusting the rate of movement of one of the bridge ends.
In U.S. Pat. No. 3,703,016 to Schramm, relative rotation of two cylindrical wheels indicates bridge misalignment and adjusts the two power supplies to correct the misalignment.
Similar mechanisms can be found in U.S. Pat. Nos. 2,932,260 to Puma, 2,601,831 to Caillard, 2,955,546 to Leibherr, 2,935,032 to Tingskog and 2,974,606 to Macrawder as well as Italian Pat. No. 575,727 and German Pat. Nos. 1,109,338 and 1,120,658.
The suitability and acceptance of some prior art anti-skewing devices has been restricted due to certain disadvantages. For example, the response time of some earlier systems has been too slow for the skewing to be corrected at the time required. This could lead to the highly undesirable condition where the skewing has actually reversed by the time the correction occurs. Additionally, on those earlier systems employing mechanical devices such as rollers contacting the rails to detect skewing, the accuracy of the devices or their durability is adversely affected by accumulations of debris as commonly found at industrial sites.